1. Field of the Invention
The present invention relates to a welding control apparatus and more particularly to a welding control apparatus and method used for arc welding with a single carbon dioxide gas or a mixed gas mainly containing the carbon dioxide gas being used as a shielding gas.
2. Description of the Related Art
Hitherto, a MAG welding process using an Ar-(5-30%)CO2 mixed gas as a shielding gas has been adopted in a wide range of fields because fine-grained droplets enable reduction in generation of spatters. In particular, in such a field as requires high-quality welding, an application of a pulse MAG welding process has been spreading. This process outputs a welding current in the form of pulse current with a frequency of about 100 to 350 Hz to thereby supply one droplet at each pulse.
However, since an Ar gas is more expensive than a carbon dioxide gas, a single carbon dioxide gas or a mixed gas mainly containing the carbon dioxide gas is often used as a shielding gas in a general welding process.
On the other hand, if a single carbon dioxide gas or a mixed gas mainly containing the carbon dioxide gas is used as a shielding gas, a droplet is made coarse and irregularly vibrates or deforms due to an arc force as compared to the MAG welding process. This causes a problem that short-circuit to the base metal or interruption of an arc tends to occur, droplet movement becomes irregular, and a large amount of spatters or fumes is generated.
To overcome such a problem, a method for supplying one droplet at each pulse even in carbon-dioxide-gas arc welding is proposed. This method is achieved by applying pulse welding to carbon-dioxide-gas-shielded arc welding in Japanese Unexamined Patent Application Publication Nos. 7-290241 and 7-47473, achieved by prescribing pulse parameters in Japanese Unexamined Patent Application Publication No. 7-290241, and achieved by prescribing pulse parameters and wire components in Japanese Unexamined Patent Application Publication No. 7-47473. This conventional method forms a sufficiently large droplet at the tip end of wire prior to application of a peak current to accelerate constriction of the droplet by means of electromagnetic pinch force to allow the droplet to separate from wire before an arc force presses the droplet back to the wire.
In addition, Japanese Unexamined Patent Application Publication No. 8-267238 discloses another example of the carbon dioxide gas-shielded arc welding method. This welding method switches external characteristics under control as a method of controlling an output power of a welding power source to thereby enable further reduction of spatters.
U.S. Pat. No. 5,834,732 discloses an output control device for a pulse arc welding apparatus using a shielding gas mainly containing a carbon dioxide gas, which detects separation of a droplet by detecting an increase in voltage or resistance and reduces a current amount for a predetermined period from the detection to thereby suppress spatters.
Moreover, according to a technique disclosed in U.S. Pat. No. 6,037,554, spatters are suppressed with a pulse arc welding device using a shielding gas mainly containing a carbon dioxide gas, which outputs two different kinds of pulse waveforms, that is, a first pulse with a pulse period and a base period being set short in accordance with increase in wire amount supplied and a second pulse with a pulse period shorter than that of the first pulse.
Further, the inventors of the present application have proposed a pulse arc welding method for alternately outputting two kinds of pulse waveforms different in pulse peak current level per period with a single carbon dioxide gas or a mixed gas mainly containing the carbon dioxide gas being used as a shielding gas. This method supplies one drop at each period as well as adjusts, even if a distance between a contact tip and a base metal is changed, at least one of a peak current (Ip2), a base current (Ib2), a peak period (Tp2), and a base period (Tb2) of a second pulse used to shape a droplet while maintaining an orderly operation of supplying one droplet at each period to thereby keep a constant arc length (see US Patent Unexamined Application Publication No. 2007/0210048).
Referring to FIG. 14, the pulse arc welding method disclosed in US Patent Unexamined Application Publication No. 2007/210048 is described. FIG. 14 is an explanatory view schematically showing how a welding wire tip end portion is changed over time based on a pulse waveform generated by the pulse arc welding method disclosed in US Patent Unexamined Application Publication No. 2007/0210048. A conventional welding control apparatus for performing the pulse arc welding method alternately generates two kinds of pulse current (pulse signals) different in pulse waveform, more specifically, a first pulse 901 and a second pulse 902 as shown in a lower portion of FIG. 14 and then outputs the generated pulses to a welding power source. Here, pulse parameters of the first pulse 901 and the second pulse 902 are set to satisfy predetermined conditions.
If pulse arc welding is carried out under such conditions, a wire tip end 905 of welding wire (hereinafter simply referred to as “wire”) that induces an arc 904 between the wire and a welding material (not shown) is constricted into a droplet and then drops as shown in an upper time-series diagram of FIG. 14. First of all, 911 indicates a state of a droplet from a second pulse peak period in which the droplet grows at the wire tip end 905 after separation of a droplet formed in a previous pulse period up to late second pulse base period. At this time, since a current value rapidly decreases from a second pulse peak current to a second pulse base current, an upward force is weakened at the wire tip end 905, and the droplet is shaped to hang down at the wire tip end 905 as indicated by 911.
Subsequently, in a first pulse peak period, the droplet rapidly separates by means of an electromagnetic pinch force generated by a peak current in the wire while deforming to form a constriction 906 as indicated by 912. When detecting the separation of the droplet 907 from the wire tip end 905, the welding control apparatus rapidly decreases a current value from a current measured upon the detection to a first pulse base current. Then, at the instance when the arc is moved toward the wire from which the droplet was separated, the apparatus shifts a current period to the first pulse base period as indicated by 913 to thereby substantially reduce a current value. As a result, it is possible to considerably reduce an amount of small spatters generated due to flying spray of the wire at the constriction 906 or flying spray of residual melt after separation.
After that, in the second pulse peak period, the welding control apparatus sets a second pulse peak current to such a level that causes neither release nor flying spray of melt residual in the wire from which the droplet was separated, and then allows a droplet to grow in accordance with the second pulse as indicated by 914. Then, in the second pulse base period, the welding control apparatus shapes the droplet as indicated by 915. Following this, the process for shaping the droplet starts again from the state 911. Thus, the welding control apparatus can supply one droplet at each period with clockwork regularity under normal conditions.
All of the methods disclosed in Japanese Unexamined Patent Application Publication Nos. 7-290241, 7-47473, and 8-267238 can supply one droplet at each pulse to enhance regularity for droplet supply although using an inexpensive carbon dioxide gas as a shielding gas as well as can reduce an amount of large spatters as compared with a welding method using no pulse. However, the methods disclosed in these publications prompt a droplet to separate during the pulse peak period, leading to a problem of generating a large amount of small spatters due to flying spray of wire at the constriction of the wire tip end portion upon the separation of a droplet or due to flying spray of melt residual in the wire after the separation of a droplet.
The output control device disclosed in U.S. Pat. No. 5,834,732 can suppress generation of spatters by lowering a current for a predetermined period from detection of droplet separation. However, according to this method, since a pulse peak current is constant at every pulse irrespective of whether melt is released, if the pulse peak current is set so high that a droplet can separate, molten metal residual in wire from which the droplet was separated might be sprayed by means of a strong arc force upon the next application of a pulse peak current after the separation to generate large spatters. Moreover, since the wire is overheated too much upon formation of a droplet, a large amount of fumes is generated. If a pulse peak current is set lower to suppress such generation of fume, a droplet could not separate in a pulse peak period.
Moreover, the pulse arc welding method disclosed in U.S. Pat. No. 6,037,554 can reduce spatters by outputting two kinds of pulse waveforms including a first pulse with a pulse period and a base period being set short in accordance with increase in wire amount supplied and a second pulse with a pulse period shorter than that of the first pulse. However, if a first pulse period and a first base period are set short in accordance with increase in wire amount supplied, a droplet shape at the wire tip end cannot be adjusted prior to application of an electromagnetic pinch force generated by a second pulse and the electromagnetic pinch force cannot effectively act thereon. As a result, it is difficult to supply one droplet at each period, leading to a problem of generating large spatters.
Moreover, the method disclosed in US Patent Unexamined Application Publication No. 2007/210048 alternately outputs two kinds of pulse waveforms different in pulse peak current level per period, making it possible to considerably reduce an amount of small spatters or large spatters sprayed upon the next application of pulse peak current after the separation of a droplet and in addition, supply one droplet at each period with a wide range of wire supplying rate. However, there is a problem that if a first pulse fails to separate a droplet due to any disturbance, a regularity of droplet supply is disturbed from then on and several periods are required to return to a normal state. Thus, spatters and fumes increase during that period.